The present invention relates to an apparatus for positioning prosthetic cup assemblies or parts thereof in a patient. More particularly, the present invention relates to gripping head useful for holding an acetabular cup assembly while the acetabular cup assembly is inserted into position in a hip of a patient.
Prosthetic cup assemblies that replace diseased, damaged, or degraded bone are known. In one common operation, the acetabulum of a hip is partially replaced with an acetabular cup assembly that includes a metal shell component for attachment to an acetabulum to replace the natural socket. A polymeric bearing component is disposed in the metal shell component to provide a hemispherical bearing surface for receiving a femur ball prosthesis element. Often, the polymeric bearing component is nonsymmetrical, having a built-up lip around a portion of the hemispherical bearing surface to help prevent dislocation of an installed femur ball from the hemispherical bearing surface. In addition, the bearing can be inserted into the shell after the shell is already in place in the acetabulum. Additionally, a one-piece polymeric acetabular component can be cemented into the cavity without any accompanying metal shell.
Proper positioning of the acetabular cup assembly usually requires reaming of the acetabulum to define a suitable bone cavity, followed by implantation of the shell component and, in some cases, subsequent fixation of the bearing component to the shell component. During installation of the hemispherical bearing component, the surgeon selects an orientation of the bearing with respect to the shell component to align the lip of the nonsymmetrical bearing component in the most advantageous position to reduce the likelihood of dislocation of the femur ball. A positioning/impactor device is necessary to hold the bearing component at a selected orientation with relation to the shell component, and allow a orthopaedic surgeon to drive the bearing component into attachment with the shell component.
The present invention provides for an apparatus for positioning a bearing component of a prosthetic acetabular cup assembly in a patient's hip. Typically, the bearing component includes an inner bearing surface for receiving a femoral ball and an outer surface attachable to a shell component. The shell component, which may be found of generally composed of titanium and have a bone growth promoting outer surface, is attached to an acetabulum to replace a natural hip socket. In one embodiment, the shell component also includes an inner surface defining a cavity for receiving the outer surface of the bearing component therein. The positioning apparatus includes a handle attached to a split head. The split head can be attached to extend parallel and collinear with respect to the handle, or can alternatively be attached so that it extends at a some predetermined angle relative to the longitudinally extending handle. The latter attachment position of the split head is particularly valuable for use in conjunction with asymmetric bearing components, and the former for use with symmetric bearing components. The split head is divided into first and second gripping elements that jointly form a gripping surface to engage the inner bearing surface of the bearing component.
A spring is attached to one of the first and second gripping elements. The spring is biased to urge separation of the first and second gripping elements. A lever arm attached to one of the first or second gripping elements can be moved to oppose the spring bias and move the first and second gripping elements closer together to allow positioning of the split head in contact with the inner surface of the bearing component.
In preferred embodiments the split head is configured to present a generally hemispherical surface. The first and second gripping elements are formed by hemisection (division in half) of the hemisphere, providing two gripping elements that are substantially mirror images of each other. Encircling the split head is a first gripping ridge defined to promote frictional engagement of the split head with the inner bearing surface of the bearing component. Optionally, additional gripping ridges can defined in the split head to promote engagement of the split head with differently sized bearing components, whether larger or smaller.
These ridges, as will be more fully described hereinafter, are proportioned to hold the plastic bearing component for forcible insertion into the affixed shell. Preferably, the plastic bearing component, or the assembled plastic bearing component and shell component, are held so that the plastic bearing component is not stressed during insertion into an acetabular bone cavity.
Typically, the spring is biased to hold the gripping elements apart from each other. Before the split head can be inserted to contact the inner bearing surface of the bearing component, the spring biasing force must be overcome to bring the gripping elements together. In preferred embodiments, the first gripping element of the split head is fixedly held relative to the handle, and the second gripping element is movable, usually by pivot action about a pivot pin, with respect to the handle. A moving means, such as a lever arm is attached to the second gripping element of the split head, and movement of the lever arm facilitates movement of the second gripping element toward the immobile first gripping element.
Another aspect of the present invention relates to the coupled attachment of the split head to the handle. In a preferred embodiment, the split head is provided with internal threads capable of threadingly engaging external threads defined on the handle. In other embodiments, a declination adaptor can be used to angle the split head with respect to the handle. A body of the declination adaptor includes internal threads to engage the external threads of the handle, and external threads to engage the internal threads of the split head. The external threads of the body are directed at a non-parallel angle relative to the internal threads of the body, in effect canting the attached split head relative to the handle. This is particularly useful for insertion of asymmetric bearing components, since a declination adaptor with an appropriate angle can be selected to match the degree of asymmetry of the bearing component.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.